The foregut is constantly barraged with flora contaminating ingested substances. In order to ensure that digestion and absorption of nutrients is unimpeded, bacterial replications must be limited until the cecum is reached. Although the 'gastric acid barrier' effectively reduces bacterial counts in the stomach, many bacterial species can escape this barrier. Thus, additional defenses are in place to eradicate organisms resistant to gastric acid/pepsin. The most described secondary defenses include the antimicrobial peptides such as the defensins and cryptidins. Here, we describe for the first time in mammalian tissue, the function of surface oxidases, termed dual oxidase or DUOX, in the control of foregut bacterial colonization. Although these oxidases have been implicated in airway defense, and have been studied in drosophila and C. elegans intestine, there is no published report of their function in the mammalian gut. On the basis of our preliminary data and literature publications, we have formulated a hypothesis regarding how foregut bacteria activates DUOX through pattern recognition receptor (PRR) sensing, ATP signalling, purinoreceptor activation, and (Ca++)i increase, with H2O2 secreted into the lumen, which combined with secreted SCN- to form the strong oxidant, microbicidal anion OSCN-. This regulated system, for the secretion of microbicidal substances, which I will term the 'DUOX system', is likely to be an important foregut defense system against bacterial colonization with important pathogens. We propose to study Duox and PRR in the perfused anesthetized, rat and mouse intestine, measuring peroxide, ATP, and other released substances with standard assays. Knockout mice, when available, will be used to examine to function of individual components of the system. Since Duox is hypothesized to control foregut bacterial colonization, the proposed system may bear on several important diseases, such as small intestinal bacteria overgrowth (SIBO), cystic fibrosis, and inflammatory bowel disease.